avocado.multiplexer: Support for multiple yaml files [v1]

Hi guys, this pull request implements similar behavior as https://github.com/avocado-framework/avocado/pull/271 only on top of the direct yaml parsing.

First couple of commits are just bugfixes and optimization. The unittests are adjusted as last commit and it covers 98% of the multiplexer and tree.

Regards,
Lukáš